Creation of Boron Vacancies in Hexagonal Boron Nitride Exfoliated from Bulk Crystals for Quantum Sensing

Boron vacancies (VB–) in hexagonal boron -nitride (hBN) have sparked great interest in recent years due to their optical and spin properties. Since hBN can be readily integrated into devices where it interfaces a huge variety of other 2D materials, boron vacancies may serve as a precise sensor which can be deployed at very close proximity to many important materials systems. Boron vacancy defects may be produced by a number of existing methods, the use of which may depend on the final application. Any method should reproducibly generate defects with controlled density and desired pattern. To date, however, detailed studies of such methods are missing. In this paper, we study various techniques for the preparation of hBN flakes from bulk crystals and relevant postprocessing treatments, namely, focused ion beam (FIB) implantation, for creation of VB–s as a function of flake thickness and defect concentrations. We find that flake thickness plays an important role when optimizing implantation parameters, while careful sample cleaning proved important to achieve consistent results.


Sample Preparation -Hot plate
As mentioned in the main text, some samples were cleaned using a hot plate set to 400 • C for 15 minutes.This process, while less controlled than the forming gas heating, is widely available and straightforward to implement, and thus of interest.
In Figure S1 we present PL spectral measurements performed on flake N3, which was prepared using the hot plate method and ex-Figure S1: Flake N3 dose test, varying fluences in the range of 10 13 ∼ 10 14 cm −2 posed to a nitrogen FIB dose test.These results indicate efficient creation of VB − defects, comparable to the yield obtained using forming gas sample preparation.The hot plate cleaning process did not eliminate all contaminants from the flakes, thus some fluorescence is visible for wavelengths below 700 nm, similarly to flakes that went though annealing in forming gas.
We propose that the contaminants might have been introduced during a SEM imaging session, though this statement was not thoroughly tested and cannot be fully confirmed.
2 Defect creation -Additional Techniques

Bulk Ion Implantation
Bulk ion implantation was attempted using standard, commercial (semiconductor industry) implanter instrumentation (Innovion Corp.) to reproduce previously reported results.S1 In our case these trials were not successful, resulting in measured fluorescence but not with characteristics matching those of VB − s (Fig. S2).We attribute the measured fluorescence to other defects.Nevertheless, we note that for this process no cleaning was performed on the flakes post transfer to the Si substrate (before implantation).Since unclean flakes exhibited limited defect creation also using FIB, we deduce that sample cleaning before implantation plays an important role.

Electron Irradiation
Furthermore, we studied electron irradiation instead of ion implantation, performed using two electron sources: e-beam lithography (100keV, Elionix ELS-G100) and TEM (80keV and 300keV, Themis-Z).The hBN samples used for these electron irradiation studies went through the same sample preparation as detailed for the FIB.For TEM irradiation, the flakes were transferred to TEM Gold grids (200 Mesh) with carbon film (R 1.2/1.3)using the following procedure: first, hBN flakes were transferred to a Si substrate spin coated with Poly(vinyl alcohol) (PVA), then the TEM grid's carbon side was placed on the Si, and finally a drop of water was added to dissolve the PVA, allowing the hBN flakes to be transferred to the grid.Electron irradiation under the above conditions did not result in successful creation of VB − s, as can be seen in Fig. S3 for e-beam and in Fig. S4 for TEM.We attribute this to insufficient energy of the electron beam.Theoretically, the maximum energy that 80 keV and 100 keV electrons can transfer to B atoms are approximately 17.5 eV and 22.2 eV, respectively.S2,S3 Given the displacement threshold of 19.36 eV reported for B in pristine hBN, S4 it is unlikely to create VB − s in hBN flakes using 80 keV electrons.Although the energy transfer from 100 keV electrons should be sufficient for VB − s creation, the probability of such a process is relatively low and likely requires very high electron beam doses.S4

Data smoothing and SNR
The PL spectral data was smoothed using standard low-pass filtering, for enhanced presentation clarity and for quantitative SNR and VB yield analysis.The raw data was smoothed out using a triangular low-pass filter as follows: where S(λ) is the output, smoothed signal, ∆λ = 0.2 nm is the wavelength increment, Y (λ) the raw data and w = 100 is the size of the smoothing filter.
The SNR of the VB − signal, proportional to the VB − yield, is estimated from the data, by fitting a Gaussian to the raw PL spectral data (see Fig. S5) using the following formula: We excluded data points below 650 nm, since the relevant data is above this threshold, and technically a high-pass filter at that wavelength was used during acquisition.
Finally for the SNR calculation, we extract the ratio between the area under the Gaussian and root mean square error (σ RM SE ) of the fit: 4 Additional Data

FIB Fluence and Thickness Dependence
We perfromed additional experiments to further verify our results, including dose tests and thickness dependence studies, analyzing the PL spectral signatures from the various implanted regions as described in the main text.In Fig. S6 we present SEM and confocal images of a flake implanted with nitrogen FIB dose test.Such studies were performed on several additional flakes of varying thicknesses.As the dose of ∼ 10 14 [cm −2 ] proved most efficient in this range, we analyzed the VB − yield for this dose as a function of thickness, as presented in Fig. S7.These results are consistent and complement the data and analysis presented in the main text.

Optically Detected Magnetic
Resonance (ODMR) In addition to the magnetic resonance measurements presented in the main text, we have also characterized the created VB − defects in the oxygen FIB samples.A representative ODMR measurement is shown in Fig. S8, demonstrating the signature resonance feature of VB − defects.
Figure S8: ODMR measurement performed on flake O1, implanted with oxygen FIB, without a bias magnetic field.The magnetic resonance signature of VB − is clearly visible.

Figure S3 :
Figure S3: Two flakes that underwent e-beam dose test ranging from 10 17 cm −2 to 10 19 cm −2 at 100 keV.(a) Microscope image of the flakes.(b) Confocal scan of the fluorescence from the flakes after the irradiation.(c) PL Spectra of exposed areas on the flakes, depicting no VB − signature.

Figure S4 :
Figure S4: Two flakes that underwent TEM dose test ranging from 10 19 cm −2 to 10 22 cm −2 at 300 keV.(a) Microscope image of the flakes.(b) Confocal scan of the fluorescence from the flakes after the irradiation.(c) PL spectra of exposed areas on the flakes, depicting no VB − signature.

Figure S6 :
Figure S6: Additional dose test studies using nitrogen FIB, presented on 110 nm thick flake.(a) SEM image.(b) Confocal scan depicting the PL from the different implant regions.